Apparatus and method for providing an adjustable positive stop in space

ABSTRACT

A method and apparatus are disclosed for constraining motion of an end-effector in space. In one embodiment, the apparatus may include a mechanical positioner and first and second stops. The first and second stops may be controllable by a drive mechanism to constrain movement of the mechanical positioner and thereby to constrain the ability of a user to manipulate an end-effector outside a predetermined range of motion. The first and second stops may be further controllable by the drive mechanism to permit movement of the end-effector within the predetermined range of motion.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application claims priority to U.S. Prov. Ser. No.60/877,412, entitled “Apparatus and Method for Providing an AdjustablePositive Stop in Space,” filed Dec. 27, 2006, hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to the field of robots and morespecifically to the field of haptic robots.

BACKGROUND OF THE INVENTION

Haptic interfaces permit a user to experience a sense of touch in avirtual or haptic environment. Such interfaces are finding acceptance invirtual reality games and in performing tasks that are virtually imaged.One area which uses haptic interfaces to help a user perform a task iscomputer aided surgery.

In computer aided surgery, a haptic interface can be used to providehaptic guidance to a surgeon. For example, a surgical instrument, suchas a cutting tool, can be coupled to a haptic interface. The hapticinterface may be, for example, part of a robotic device, such as arobotic arm. As the surgeon moves the surgical instrument in real space(e.g., to cut bone or other anatomy), constraints may be imposed on thesurgeon through the haptic interface that limit his ability tomanipulate the surgical instrument. For example, the surgeon's abilityto manipulate the surgical instrument may be constrained so that thesurgeon can only move the surgical instrument within a defined cuttingregion. The constraints may be based, for example, upon a desiredrelationship between the surgical instrument and the defined cuttingregion in real space. This real space relationship can be represented ina virtual environment as a relationship between a virtual instrument anda haptic object in virtual space, where the virtual instrumentcorresponds to the physical surgical instrument and the haptic objectcorresponds to the defined cutting region. In operation, the surgeonmanipulates the surgical instrument while it is attached to the end ofthe haptic interface. Constraint feedback is provided to the surgeonthrough the haptic interface, which imposes a force on the surgeonsufficient to maintain the desired relationship between the virtualinstrument and the haptic object.

For example, the haptic object may be a virtual protective boundary foran anatomic structure or a shape that is to be cut into bony anatomy.The virtual boundary is registered (or correlated) to the anatomy of apatient, and the virtual instrument is registered (or correlated) to theactual surgical instrument. To enable the surgeon to interact with thevirtual environment via the haptic interface, a haptic renderingalgorithm is employed. Haptic rendering is the process of computing andapplying forces in response to user interactions with virtual objects.Using the haptic rendering algorithm, the haptic interface may beconfigured so that the force experienced by the surgeon increases as thevirtual instrument approaches the virtual boundary. This increasingforce provides a warning to the surgeon that he is near a forbiddenregion of the workspace (e.g., an anatomic structure of interest orother boundary) and therefore should proceed with caution in order toprevent unwanted penetration into and damage to the structure (forexample, preventing a drill bit from entering too deeply into the bone).If the surgeon tries to force the instrument beyond the virtualboundary, the haptic interface provides an increasing force to preventsuch motion. In this manner, the virtual boundary functions as a hapticstop to maintain the surgical instrument within a desired region of theworkspace.

Preventing movement using a haptic stop, however, is typicallyaccomplished with an admittance or impedance based system. An admittancedevice senses forces exerted by a user and responds by changing theposition of the device (e.g., the position of the surgical instrument).Although admittance devices can provide stiff boundaries, they requireforce sensors and generally feel heavy to the user as the user moves thedevice through free space. In contrast, an impedance device senses aposition of the device (e.g., a position of the surgical instrument) andresponds by applying forces to the device by applying limited power toactuators of a backdrivable haptic interface system. Impedance devicesgenerally feel relatively light to the user when moved through freespace and are preferable for certain applications where the users wantsto have a relatively light motion and to feel interaction forces withreal objects when moving in free space. The output force of theactuators, however, is finite, and impedance devices are not able togenerate boundaries as stiff as those generated by admittance devices.As a result, it may be possible for the surgeon to overcome theconstraints imposed by the actuators and force the surgical instrumentpast the virtual boundary or haptic stop. This would result in unplanneddamage to the tissue being operated upon.

What is needed is a haptic device having an adjustable positive stopthat can provide haptic constraint forces sufficient to preventerroneous movements by a user of the haptic device while stillpermitting the user to experience flexibility of motion during operationof the haptic device.

SUMMARY OF THE INVENTION

One example embodiment of the invention relates to an apparatuscomprising a mechanical positioner and first and second stops. The firstand second stops are controllable by a drive mechanism to constrainmovement of the mechanical positioner and thereby to constrain theability of a user to manipulate an end-effector outside a predeterminedrange of motion. The first and second stops are further controllable bythe drive mechanism to permit movement of the end-effector within thepredetermined range of motion.

Another example embodiment relates to a method for constraining movementof an end-effector in space. The method comprises permitting a linkagejoint to move freely when the end-effector is positioned a distancegreater than a first predetermined value from a predetermined locationin space. The method further comprises permitting the linkage joint tomove freely within a predetermined range of motion defined by first andsecond stops when the end-effector is positioned a distance less than orequal to the first predetermined value and greater than a secondpredetermined value from the predetermined location in space. The methodfurther comprises constraining the linkage joint from moving in aspecific direction by one of the first and second stops when theend-effector is positioned a distance approximately equal to or lessthan the second predetermined value from the predetermined location inspace. The constraining of the linkage joint constrains the movement ofthe end-effector in space.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.The advantages of example embodiments of the invention described above,together with further advantages, may be better understood by referringto the following description taken in conjunction with the accompanyingdrawings. In the drawings, like reference characters generally refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of example embodiments of the invention.

FIG. 1 is a schematic diagram of a positive stop haptic system of anembodiment of the invention;

FIGS. 2a-2c are diagrams depicting the positive stop of FIG. 1 under avariety of conditions of a haptic interaction point moving in virtualspace;

FIG. 3 is another schematic diagram of an embodiment of the invention ofFIG. 1;

FIG. 3a is a schematic diagram of another embodiment of the inventionhaving independent distal and proximal positive stops;

FIG. 3b is a schematic diagram of another embodiment of the inventionhaving both micro and macro resolution drive mechanisms;

FIG. 3c is a schematic diagram of another embodiment of the inventionillustrating a mechanical positioner and positive stops having differentshapes;

FIG. 4 is a schematic diagram of an embodiment of a plurality of thepositive stop haptic systems of FIG. 1 arranged to constrain anend-effector;

FIG. 5 is a schematic diagram of another embodiment of a plurality ofthe positive stop haptic systems of FIG. 1 arranged to constrain anend-effector; and

FIG. 6 is a schematic diagram of a positive stop haptic system of anembodiment of the invention utilizing rotary motion.

FIG. 7 is a schematic diagram of a surgical system of an embodiment ofthe invention.

FIG. 8 is a perspective view of an anatomy tracker.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, in various applications, when a tool or an instrument isbeing used, it is desirable to limit the range of possible movement ofthe tool. Limiting tool movement can prevent damage to a workpiece, forexample, by preventing a user of the tool from cutting too deeply intothe workpiece. When the user is operating a tool coupled to a roboticdevice in a particular process, such as surgery, manufacturing, or otherapplications, the limitations on tool movement can be implemented usingphysical stops.

In part, exemplary embodiments of the invention described herein includerobotic devices that use different physical stops and controls suitablefor constraining movement of a tool within a prescribed range of motion.This is accomplished by controlling the motion of different linkages andmembers of the robotic device such that for a particular range of motionor distance, parts of the device are allowed to travel freely while forother prescribed ranges of motion or distances, motion of the parts ofthe device is constrained in speed or halted altogether. Thus, thephysical stops can be controlled to permit a user to move the tool in adesired region of a workspace and to constrain the ability of the userto manipulate the tool into a forbidden region of the workspace. Usingactuators and tracking devices in combination with mechanical linkagesand joints make such systems possible.

The exemplary embodiments also enable real time or dynamic positioningof a physical stop or constraint relative to a moving object. That is,for a moveable workpiece, such as a machineable block or a patient's hipbone, the example system described herein is able to respond to themotion of the workpiece by adjusting the physical stops in responsethereto. This allows a surgeon or a worker to perform an operation onthe patient or workpiece with the safety associated with the physicalstops in spite of the movement of the patient or workpiece.

For example, in a medical application for a haptic device, the virtualenvironment created by a haptic rendering process includes virtual (orhaptic) objects (such as guidewires, implant models, or protectiveboundaries) that interact with a virtual representation of a medicalinstrument. The virtual medical instrument is linked (or registered) tothe physical medical instrument so that movement of the physical medicalinstrument results in corresponding movement of the virtual medicalinstrument. Similarly, the virtual object is linked (or registered) tothe patient's physical anatomy so that movement of the physical anatomyresults in corresponding movement of the haptic object. Because thevirtual and physical environments are registered or associated with oneanother, the user's manipulation of the medical instrument isconstrained based on interaction between the virtual object and thevirtual representation of the medical instrument, which typically makesuse of a point in virtual space termed the haptic interaction point(HIP). The HIP is a point in virtual space that corresponds to aphysical point on the medical instrument, for example, the tip of asurgical burr or drill bit. As the HIP moves through virtual space, thehaptic rendering algorithm computes forces based on a desiredrelationship between the HIP and the virtual object, such as arelationship where the boundaries of the virtual object define apermissible workspace (e.g., a desired region of the workspace or apredetermined range of motion in the workspace) and the HIP ismaintained within the boundaries of the virtual object. The actuators ofthe haptic device generate the computed forces, and the haptic interfacetransmits these forces to the user in an effort to maintain the desiredrelationship. Thus, as the surgeon moves the physical instrument, he orshe feels the forces that the HIP experiences in virtual space. In thisway, movement of the medical instrument can be constrained, for example,to keep the medical instrument within a cutting boundary defined by thevirtual object and to prevent the medical instrument from moving into aforbidden region of the workspace. Thus, the user is permitted to movethe tool within the predetermined range of motion of the workspace, butthe ability of the user to manipulate the tool outside the predeterminedrange of motion is constrained.

Thus, the movement of the HIP can be constrained to limit the way thesurgeon can move the physical medical instrument. For example, thevirtual object can be a virtual guidewire which constrains the HIP tomove along a specific path. Attempted deviation from the path results inforces being generated that prevent or at least reduce the ability ofthe HIP to deviate from the path. These forces are transmitted to thesurgeon through the haptic interface and thereby guide the physicalinstrument along a guide path in physical space in order to avoidanatomic features as the instrument is moved to a predefined location.In another example, the virtual object can be a virtual model of animplant to be implanted in a bone of a patient during a jointreplacement operation. The virtual implant model is associated with (orregistered to) the physical bone of the patient and defines the portionof bone to be removed so that a physical implant can be installed, asdescribed, for example, in U.S. patent application Ser. No. 11/357,197,U.S. Pub. No. 2006/0142657, filed Feb. 21, 2006, and hereby incorporatedby reference herein in its entirety. In the virtual environment, the HIPis constrained to stay within the cutting boundary defined by theimplant model (i.e., the desired region of the workspace). If the HIPattempts to violate the boundary and to move into the forbidden regionof the workspace, forces are transmitted to the surgeon through thehaptic interface to prevent or at least reduce the surgeon's ability toviolate the cutting boundary with the physical instrument.

Although the forces applied to the HIP in virtual space can becomesubstantially infinite to prevent the HIP from progressing beyond apredefined point in virtual space, in a conventional haptic device, this“infinite” force is applied by the actuators and so in actuality willresult in a less than infinite force being applied to the user throughthe haptic interface. As a result, the user, either intentionally orinadvertently, may overcome the constraint force to deviate from a guidepath or violate a haptic boundary.

According to an embodiment of the present invention, a moveable positivestop is provided. One advantage of the moveable positive stop is thatthe positive stop can provide a sufficiently strong constraint to limittool movement while still allowing the surgeon to retain flexibility ofmotion when moving the tool. FIG. 1 shown an embodiment of a positivestop haptic system 10 constructed in accordance with an exemplaryembodiment of the invention. The positive stop haptic system 10 includesa drive mechanism 50, a mechanical positioner 30, an actuated linkagejoint 26, a non-driven or passive linkage joint 34, a proximal positivestop 18, and a distal positive stop 22. The actuated linkage joint 26and the passive linkage joint 34 form a prismatic joint assembly whichpermits relative linear motion between the actuated linkage joint 26 andthe passive linkage joint 34.

The drive mechanism 50 is located at one end of the actuated linkagejoint 26, and the positive stops 18 and 22 are located at the other endof the actuated linkage joint 26. As shown in FIG. 1, the distancebetween the positive stops 18 and 22 is indicated by (2δ), and themidpoint distance between the positive stops 18 and 22 is δ. Themechanical positioner 30, which is disposed at one end of the passivelinkage joint 34, is located between the proximal positive stop 18 andthe distal positive stop 22. The other end of the passive linkage joint34 is connected to an end-effector (not shown). The end-effector may be,for example, a medical instrument, a tool, or another link.

In operation, as the end-effector coupled to the passive linkage joint34 is moved by a user (e.g., by a surgeon during a surgical procedure tosculpt bone), the mechanical positioner 30 moves between the proximalpositive stop 18 and the distal positive stop 22. When the actuatedlinkage joint 26 is fixed in place by the drive mechanism 50, thepositive stops 18 and 22 are also fixed in place and will prevent themechanical positioner 30 from moving beyond the positive stops 18 and22. In this manner, the positive stops 18 and 22 define a predeterminedrange of motion for the mechanical positioner 30 (and thus theend-effector) and function to permit movement of the end-effector withinthe predetermined range of motion and to constrain the user's ability tomove the end-effector outside the predetermined range of motion. Whendesired, the drive mechanism 50 can be actuated to move the actuatedlinkage joint 26, which results in movement of the positive stops 18 and22 toward or away from the drive mechanism 50. In this manner, the drivemechanism 50 is controllable to adjust the positive stops 18 and 22 toadjust the predetermined range of motion, such as to adjust the positionin space of the predetermined range of motion. To enable free motion ofthe end-effector, the positive stops 18 and 22 can be configured to movewhen the mechanical positioner 30 moves so that the mechanicalpositioner 30 does not contact either the proximal positive stop 18 orthe distal positive stop 22. This enables the user to move theend-effector freely because the mechanical positioner 30 has completefreedom of motion along the axis of the passive linkage joint 34. Toenable the positive stops 18 and 22 to move with the mechanicalpositioner 30, the system can include a sensor to detect motion of themechanical positioner 30 and a controller to control the drive mechanism50 to move the actuated linkage joint 26 so that the positive stops 18and 22 track along with the mechanical positioner 30 and thus do notcontact the mechanical positioner 30. In this manner, the positive stops18 and 22 can be adjusted in response to movement of the mechanicalpositioner 30. To trigger a physical stop (e.g., when the end-effectoris near a forbidden region of the workspace), the drive mechanism 50 ishalted, thereby preventing further motion of the positive stops 18 and22. When the mechanical positioner 30 contacts one of the positive stops18 and 22, the user's motion of the end-effector is constrained. In thismanner, the positive stop haptic system 10 provides a moveable positivestop that can constrain the end-effector with a physical stop (asopposed to a stop based on forces generated by an actuator) that theuser cannot overcome while still allowing the surgeon to retainflexibility of motion when moving.

As discussed above, the position of a portion of the end-effector inphysical space (such as the tip of the tool or instrument) correspondsto the HIP in virtual space. Thus, as the instrument or end-effectormoves in real space, the HIP moves, for example, toward the virtualobject in virtual space. Referring to FIGS. 2a-2c , when an HIP 27 ispositioned a distance (d) from a portion of the virtual object 25, suchas a boundary of the virtual object 25, one of three conditions are met.First, as shown in FIG. 2a , the value of (d) may be greater than afirst predetermined value, for example, the distance between themechanical positioner 30 and one of the positive stops 18 and 22(d>(δ)). This condition is free motion. Second, as shown in FIG. 2b ,the value of (d) may be greater than a second predetermined value andless than or equal to the first predetermined value (e.g., 0<d≦(δ)).This condition is approach motion. Third, as shown in FIG. 2c , (d) maybe less than or equal to the second predetermined value (e.g., d≦0)meaning that the HIP 27 is at or within the virtual object boundary.This condition is constrained motion.

Referring to FIG. 2a , which illustrates free motion, when (d>(δ)), thatis when the HIP 27 is located at a distance (d) that is greater thanhalf the distance between the stops 18 and 22, the passive linkage joint34 moves freely. The motion of the passive linkage joint 34 is free notonly because the mechanical positioner 30 moves freely between the stops18 and 22 (arrow A), but also because the actuated linkage joint 26 isdriven by the drive mechanism 50 to track the motion of the mechanicalpositioner 30 so that the positive stops 18 and 22 also move (arrow B).The drive mechanism 50 preferably moves the actuated linkage joint 26such that the distance between the mechanical positioner 30 and theproximal positive stop 18 or the distal positive stop 22 is nominallymaintained at the midpoint distance δ.

Referring to FIG. 2b , which shows approach motion, once the HIP 27 isat a distance (d) equal to (δ), the drive mechanism 50 servos theactuated linkage joint 26 to resist further forward motion. This causesthe distal positive stop 22 and the proximal positive stop 18 to becomestationary and hence only allow the mechanical positioner 30 to movebetween the stops 18 and 22 (arrow A). Therefore, the passive linkagejoint 34 can only move a maximum distance (δ) in a direction away fromthe actuated linkage joint 26 before the mechanical positioner 30contacts the distal positive stop 22 preventing further motion. Thisthen brings the HIP 27 to the surface of the object (d=0).

Referring to FIG. 2c , which shows constrained motion, once the HIP 27has reached d=0, the mechanical positioner 30 contacts the distalpositive stop 22, preventing further motion. Thus, the system provides aphysical stop when the HIP 27 makes contact with the virtual object 25.

The example embodiments of the invention also relate to systems whereinthe actuators and movable components are linked together in varioussuitable serial or parallel configurations. That is, in a seriesconfiguration, the drive mechanism 50 may be attached to the free end ofthe passive linkage joint of another positive stop assembly. Variousmedical applications, such as orthopedic surgery, are amenable to thetechniques described herein.

Referring to FIG. 3, a physical realization of a system 10 correspondingto an embodiment of a positive stop haptic system constructed inaccordance with an exemplary embodiment of the invention includes amechanical stop assembly 14, including a proximal positive stop 18 and adistal positive stop 22, attached to the distal end of an actuatedlinkage joint 26. The system 10 also includes a mechanical positioner 30attached to the proximal end of a passive linkage joint 34. The passivelinkage joint 34 can be moved freely (within the limits set by thepositive stops 18 and 22) as long as the mechanical positioner 30 doesnot contact the proximal positive stop 18 or the distal positive stop22.

As shown in FIG. 3, the distal end of the passive linkage joint 34 isattached to a revolute joint 38. The revolute joint 38 is also connecteda distal link 42, which is connected to a second revolute joint 46. Thesecond revolute joint 46 is connected to an end-effector (not shown).

In the embodiment of FIG. 3, the proximal end of the actuated linkagejoint 26 is connected to the movable portion of a drive mechanism 50.The drive mechanism 50 includes a housing connected to a fixed base 54by way of a proximal link 58. In various embodiments shown, the drivemechanism 50 can include, but is not limited to, an actuator or motor.For example, the actuator or motor can be linear, rotational,backdrivable, or non-backdrivable. A non-backdrivable mechanism 50enhances the stiffness and margin of safety associated with the physicalstop.

In operation, as the end-effector is moved by the user, the revolutejoints 38 and 46 and the distal link 42 transmit the motion to thepassive linkage joint 34, causing the mechanical positioner 30 to movebetween the proximal positive stop 18 and the distal positive stop 22.When either of the stops 18 or 22 is reached, the mechanical positioner30 can move no further in the present direction. As a result, theend-effector is also prevented from moving further in that direction.

The position in space at which the positive stop occurs is determined inpart by the position of the actuated linkage joint 26 within the drivemechanism 50. The drive mechanism 50 is fixed solidly to the base 54 bythe proximal link 58. The drive mechanism 50 is used to position theactuated linkage joint 26 in space relative to the base 54.

Referring to FIG. 3a , in another embodiment of the present invention,rather than have the distal positive stop 22 and the proximal positivestop 18 attached to a single actuated linkage joint 26 so that they aredriven together, each positive stop 18 and 22 is attached to its ownactuated linkage joint. As shown in FIG. 3a , the distal positive stop22 is disposed on a first actuated linkage joint 26, and the proximalpositive stop 18 is disposed on a second actuated linkage joint 26′.This embodiment utilizes a second drive mechanism 50′ to drive thesecond actuated linkage joint 26′. In this embodiment, because eachactuated linkage joint 26, 26′ is driven by its own drive mechanism 50,50′, both of which are attached to the same base 54 or proximal link 58,the distance between the positive stops 18 and 22 is adjustable. Thus,not only can the location of the positive stop for the mechanicalpositioner 30 be determined (and adjusted), but the distance of travel(δ) for the mechanical positioner 30 between the positive stops 18 and22 can also be adjusted. Thus, the drive mechanisms 50, 50′ arecontrollable to adjust the positive stops 18, 22 to adjust thepredetermined range of motion, such as to adjust the distance betweenthe positive stops 18 and 22. This feature is beneficial wheninteracting with narrow constraint features that would otherwise requirerapid motion of positive stops disposed on a common linkage joint (e.g.,the actuated linkage joint 26 shown in FIG. 3) as the user alternatelycontacts the distal and proximal stops in quick succession.

Referring to FIG. 3b , in yet another embodiment, a second drivemechanism 50″ is connected to the actuated linkage joint 26 of the firstdrive mechanism 50. In this embodiment, a second actuated linkage joint26″ is driven by the second drive mechanism 50″. The second actuatedlinkage joint 26″ includes the mechanical stop assembly 14. In thisembodiment, the drive mechanism 50 and the drive mechanism 50″ havedifferent resolutions and/or different ranges of motion. Thus, forexample, the drive mechanism 50 may have a coarser granularity andlarger range of motion than the drive mechanism 50″. This embodimentpermits the coarser macro drive mechanism 50 to locate the generallocation of the positive stops 18 and 22 by determining the generallocation of the actuated linkage joint 26, and permits the finer microdrive mechanism 50″ to set the ultimate fine position of the positivestops 18 and 22. This embodiment can also be used in conjunction withthe embodiment shown in FIG. 3a to provide for adjustable positive stopswith coarse and fine positioning.

Referring to FIG. 3c , in other embodiments, as will be recognized byone of skill in the art, the form of the mechanical positioner 30 andthe positive stops 18, 22 may be altered in any manner appropriate foraccomplishing the objective of constraining the user's ability to movethe end effector. For example, in contrast to FIGS. 1-3 b, whichillustrate positive stops 18 and 22 that form a “concave” or “female”shape and a mechanical positioner 30 having a “convex” or “male” shape,the positive stop device may include a mechanical positioner 30′ havinga concave shape and positive stops 18′ and 22′ having a convex shape asshown in FIG. 3c . The geometry of the mechanical positioner 30′ and thepositive stops 18′ and 22′ can be adjusted as desired to achieve thedesired constraint.

Referring to FIG. 4, three positive stop haptic systems 10 are shownpositioned between an end-effector 62 and a base 54. The positioning ofmultiple positive stop haptic systems 10 permits constraints to beapplied over multiple degrees of freedom. As shown in FIG. 5, by placingmultiple positive stop haptic systems 10, generally six in total, inconjunction with the end-effector 62, the end-effector 62 can beconstrained to stop at an arbitrary point in physical space.

Referring to FIG. 6, an exemplary embodiment of the invention can alsobe used to constrain the end-effector using rotary motion. In FIG. 6, arevolute joint assembly is shown which permits a relative rotationalmotion between an actuated linkage joint 26′″ and a passive linkagejoint 34′″. To constrain the end-effector using rotary motion, a rotarymechanical stop assembly 14′ is attached along its axis of rotation to arotational motor 50′ by a drive shaft 70. In the embodiment shown, thecommon mount to ground or the base of another apparatus is not depicted.The rotating elements share a common axis and typically a common mount.In one embodiment, the mechanical stop assembly 14′ incorporates a notch73 in the circumference of the mechanical stop assembly 14′ to define afirst positive stop 74 and a second positive stop 78. The mechanicalstop assembly 14′ is mounted on, and coaxially with, the passive linkagejoint 34′″. The passive linkage joint 34′″ includes a mechanicalpositioner 30 that extends into the notch 73. The motor 50′ causes themechanical stop assembly 14′ to rotate, thereby positioning the notch 73at a predetermined angle so that the first positive stop 74 and thesecond positive stop 78 can be made to constrain movement of themechanical positioner 30.

The passive linkage joint 34″ is connected to an upper link (e.g., anend-effector or other link) by a distal link 82. As the upper link ismoved, the distal link 82 causes the passive linkage joint 34′ to rotateuntil the mechanical positioner 30 contacts either the first positivestop 74 or second positive stop 78. By causing the motor 50′ to rotatefrom a first position to a second position, the positions at which themechanical positioner 30 contacts the positive stops 74 and 78 ischanged thereby changing the constraint placed on the end-effector.Thus, in the embodiment of FIG. 6, the positive stop is adjustable toenable real time or dynamic positioning of the positive stop relative toa movable workpiece. Additionally, the previously described embodimentsof FIGS. 1-3 c can compensate for workpiece movement in a similarmanner. For example, by adjusting the position of the positive stops 18and 22, the location of the predetermined range of motion can be changedto correspond to a changed position of the workpiece.

One application of the present invention involving a moveable workpieceis a surgical application involving cutting or sculpting of bone, suchas orthopedic joint replacement. Referring to FIG. 7, a surgical system100 for a knee replacement procedure is shown. To detect motion of theworkpiece (i.e., a femur F and/or a tibia T), the surgical system 100includes a tracking system 118 configured to track one or more objectsduring the surgical procedure to detect movement of the objects. Thetracking system 118 includes a detection device 120 that obtains a pose(i.e., position and orientation) of an object with respect to acoordinate frame of reference of the detection device 120. As the objectmoves in the coordinate frame of reference, the detection device 120tracks the object. A change in the pose of the object indicates that theobject has moved. In response, a computing system 102 can makeappropriate adjustments to control parameters for a haptic device 112(e.g., a robotic arm) mounted on a platform 116. For example, when theanatomy (e.g., the femur F or the tibia T) moves, the computing system102 can make a corresponding adjustment to a virtual haptic object(e.g., a virtual cutting boundary) that is registered to the anatomy.Thus, the virtual cutting boundary moves along with the anatomy. As thevirtual cutting boundary moves, the position of the positive stops ofembodiments of the present invention can be adjusted accordingly, forexample, as described above. The computing system 102 includes hardwareand software for operation and control of the surgical system 100 andmay comprise a computer 104, a computer 114, a display device 106, aninput device 108, and a cart 110. Computer 114 includes haptic controlutilities and programs that enable the haptic device 112 to utilize datafrom the tracking system 118.

The tracking system 118 may be any tracking system that enables thesurgical system 100 to continually determine (or track) a pose of therelevant anatomy of the patient and a pose of a tool 124 (and/or thehaptic device 112). For example, the tracking system 118 may comprise anon-mechanical tracking system, a mechanical tracking system, or anycombination of non-mechanical and mechanical tracking systems suitablefor use in a surgical environment.

In one embodiment, the tracking system 118 includes a non-mechanicaltracking system as shown in FIG. 7. The non-mechanical tracking systemis an optical tracking system that comprises a detection device 120 anda trackable element (or tracker) that is configured to be disposed on atracked object (such as the relevant anatomy) and is detectable by thedetection device 120. In one embodiment, the detection device 120includes a visible light-based detector, such as a micron tracker, thatdetects a pattern (e.g., a checkerboard pattern) on a tracking element.In another embodiment, the detection device 120 includes a stereo camerapair sensitive to infrared radiation and positionable in an operatingroom where the surgical procedure will be performed. The tracker isconfigured to be affixed to the tracked object in a secure and stablemanner and includes an array of markers (e.g., an array S1 shown in FIG.8) having a known geometric relationship to the tracked object. As iswell known, the markers may be active (e.g., light emitting diodes orLEDs) or passive (e.g., reflective spheres, a checkerboard pattern,etc.) and have a unique geometry (e.g., a unique geometric arrangementof the markers) or, in the case of active, wired markers, a uniquefiring pattern. In operation, the detection device 120 detects positionsof the markers, and the surgical system 100 (e.g., the detection device120 using embedded electronics) calculates a pose of the tracked objectbased on the markers' positions, unique geometry, and known geometricrelationship to the tracked object. The tracking system 118 includes atracker for each object the user desires to track, such as an anatomytracker 122 (to track patient anatomy), a haptic device tracker (totrack a global or gross position of the haptic device 112), an endeffector tracker (to track a distal end of the haptic device 112), andan instrument tracker (to track an instrument held manually by theuser).

In one embodiment, an anatomy tracker 122 is disposed on the patient'sanatomy and enables the anatomy to be tracked by the detection device120. The anatomy tracker 122 includes a fixation device for attachmentto the anatomy, such as a bone pin, surgical staple, screw, clamp,intramedullary rod, or the like. In one embodiment, the anatomy tracker122 is configured for use during knee replacement surgery to track afemur F and a tibia T of a patient. In this embodiment, as shown in FIG.7, the anatomy tracker 122 includes a first tracker 122 a adapted to bedisposed on the femur F and a second tracker 122 b adapted to bedisposed on the tibia T. As shown in FIG. 8, the first tracker 122 aincludes a fixation device comprising bone pins P, a clamp 126, and aunique array S1 of markers (e.g., reflective spheres). The secondtracker 122 b is identical to the first tracker 122 a except the secondtracker 122 b is installed on the tibia T and has its own unique arrayof markers. When installed on the patient, the first and second trackers122 a and 122 b enable the detection device 120 to track a position ofthe femur F and the tibia T. U.S. patent application Ser. No.11/750,840, filed May 18, 2007, entitled “Method and Apparatus forControlling a Haptic Device,” hereby incorporated by reference herein inits entirety, provides additional details regarding the arrangementshown in FIGS. 7 and 8. Additionally, paragraphs [0063]-[0081] of theabove-referenced U.S. patent application Ser. No. 11/750,840 providedetails regarding a technique for compensating for motion of objects(patient, surgical tool, robot) during a surgical procedure withoutinterrupting operation of the surgical device during the surgicalprocedure.

Although embodiments of this invention have been described in terms of ahaptic medical application, as stated previously, this invention can beused to provide movable positive stops for other applications.

While the present invention has been described in terms of certainexample embodiments, it will be readily understood and appreciated byone of ordinary skill in the art that it is not so limited, and thatmany additions, deletions, and modifications to the example embodimentsmay be made within the scope of the invention as hereinafter claimed.Accordingly, the scope of the invention is limited only by the scope ofthe appended claims.

What is claimed is:
 1. An apparatus comprising: a mechanical positionerfor an end-effector; and first and second physical stops controllable bya drive mechanism to constrain movement of the mechanical positioner andthereby to constrain the ability of a user to manipulate theend-effector outside a predetermined range of motion, and wherein thefirst and second physical stops are controllable by the drive mechanismto permit movement of the end-effector within the predetermined range ofmotion, wherein the drive mechanism is configured to control a locationof at least one of the first physical stop and the second physical stopbased on a position of a virtual object in a virtual space, a proximallink; a distal link; and a joint assembly for controlling the relativeposition of the proximal and distal links, wherein the joint assemblycomprises an actuated linkage joint, and a passive linkage joint; andwherein the actuated linkage joint and the passive linkage joint incombination include the first and second physical stops and themechanical positioner; wherein, within the predetermined range ofmotion, the passive linkage joint moves freely to permit free movementof the end-effector; and wherein, outside the predetermined range ofmotion, the passive linkage joint is constrained from moving by at leastone of the first and second physical stops; wherein the joint assemblyforms a prismatic joint assembly which permits a relative linear motionbetween the actuated linkage joint and the passive linkage joint.
 2. Anapparatus comprising: a mechanical positioner for an end-effector; andfirst and second physical stops controllable by a drive mechanism toconstrain movement of the mechanical positioner and thereby to constrainthe ability of a user to manipulate the end-effector outside apredetermined range of motion, and wherein the first and second physicalstops are controllable by the drive mechanism to permit movement of theend-effector within the predetermined range of motion, wherein the drivemechanism is configured to control a location of at least one of thefirst physical stop and the second physical stop based on a position ofa virtual object in a virtual space, a proximal link; a distal link; anda joint assembly for controlling the relative position of the proximaland distal links, wherein the joint assembly comprises an actuatedlinkage joint, and a passive linkage joint; and wherein the actuatedlinkage joint and the passive linkage joint in combination include thefirst and second physical stops and the mechanical positioner; wherein,within the predetermined range of motion, the passive linkage jointmoves freely to permit free movement of the end-effector; and wherein,outside the predetermined range of motion, the passive linkage joint isconstrained from moving by at least one of the first and second physicalstops; wherein the joint assembly forms a revolute joint assembly whichpermits a relative rotational motion between the actuated linkage jointand the passive linkage joint.